Waveguides are structures which are used to conduct electromagnetic radiation from point to point, much as wire conducts electric current. In an optical waveguide, this electromagnetic radiation is light in either a narrow or broad range of wavelengths which may be contained in the visible spectrum, or the invisible spectra such as ultraviolet or infrared.
All forms of optical waveguides have as a waveguiding medium a material of high refractive index imbedded in a medium of lower refractive index. As an example, a glass fiber of refractive index 1.45, suspended in a vacuum or air of refractive index 1.0, will act as an optical waveguide. More usually, such waveguides are clad with a material, necessarily of lower refractive index, to protect them from ambient conditions. Foreign material in contact with an unclad waveguide will reduce its transmission efficiency by scatter of the waveguided light out of the waveguiding medium, and thus the need for cladding. An example of a clad waveguide is a germania (GeO.sub.2) - doped silica (SiO.sub.2) glass fiber coated with a layer of silica glass (SiO.sub.2) for which there is a 1% difference in refractive index between the core and cladding.
High temperature waveguides are commonly made of sapphire, a crystal form of the high melting point oxide Al.sub.2 O.sub.3 (melting point 2054.degree. C.). Optical waveguides of sapphire have significant optical loss due to the lack of a suitable cladding material. A metal overcoat is used to protect such waveguides from the environment, but the transmission efficiency of this structure is low. A low loss optical waveguide requires a higher refractive index core surrounded by a lower refractive index cladding, and this is not provided in the metal-clad sapphire core waveguides.
P.J. Chandler et al. [P.J. Chandler et al., Electron. Lett. 25, 985 (1989)] have used an ion-implantation technique to produce a slab waveguide in the aluminum garnet (Y,Nd).sub.3 Al.sub.5 O.sub.12. This ion-implantation technique, unlike the technique of the present invention, makes use of the displacement of atoms in the crystal from their usual positions in the crystal lattice to generate regions of a small refractive index change. This ion-implantation technique is not suitable for use in high temperature waveguides, since the crystal structure will relax to its equilibrium state after exposure to high temperature.
P. K. Tien et al. [P. K. Tien et al., Appl. Phys. Lett. 21 (5), 207-209 (1972)]have described optical waveguides in gallium garnet and iron garnet films; specifically, Eu.sub.3 Ga.sub.5 O.sub.12 on Gd.sub.3 Sc.sub.2 Al.sub.2 O.sub.3 substrates and Y.sub.3 Fe.sub.4.3 Sc.sub.0.7 O.sub.12 on Gd.sub.3 Ga.sub.5 O.sub.12 substrates. The authors disclose that the aluminum and gallium rare earth garnets are transparent in the entire visible spectrum, and thus suitable for optical waveguides. Unsubstituted aluminum garnets such as yttrium aluminum garnet (YAG) were not considered by the authors as being suitable materials for waveguides. Neither clad waveguides nor specific compositions of epitaxial aluminum garnets as waveguiding compositions on aluminum garnet are suggested.
In an ideal waveguide, linear polarization of guided light would be maintained. However, in the real world, polarization in an actual waveguide changes, so that light that has traveled some distance in the waveguide emerges unpolarized. For some fiber optic sensors and advanced communication systems, it is required that the optical waveguides have polarization preserving properties. There are two types of polarization-sensitive single-mode waveguides. One type is a true single-polarization waveguide that can transmit light in one linear polarization but not in the other. The other type is birefringent and thus polarization-maintaining; that is, it maintains the polarization of the light that originally entered the waveguide by isolating the two orthogonal polarizations from each other while they travel down the same single-mode guide.
It is an object of the present invention to provide high temperature waveguides having a lower refractive index cladding, which waveguides further are birefringent and polarization maintaining.